of UCI
University California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
Open Shortest Path First (OSPF): General Idea: Part (A): Within a small network called an "Autonomous System" (AS), each router sends their address and an estimate of delays on outgoing links to all their neighbors. Each router also forwards information received to all outgoing links not equal to incoming link, subject to a maximum number of hops -- "flooding" Part (B): Each router uses the information it obtains, and an algorithm to find the shortest path to each possible destination in the network.
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
Part (A) in more detail: A router estimates delay to all adjacent routers, e.g. queueing time + transmit time + propagation time. The router sends this information to all adjacent routers as a message: source msg. # link 1 delay 1 ... link m delay m
When a router receives a message, if it is new it - updates its information. - copies the message to all adjacent routers (other than incoming link).
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
Part (B) in more detail: Explore links from each router & keep track of best routes found so far.
IP : OSPF
Link length
3 3 6 1 4 1 1
(1/8)
Link along shortest path from source to node Minimum distance from source to that node
3 1 4 8 0 3 6 1 1 4 1 2 3 1 4 6 1 4 8 7
Source
Link not along shortest Tree of shortest paths from souce path from source to node to other nodes
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
IP : OSPF
3 3 6 1 Source 4 1 1
(2/8)
Destination 1 4 8 0 3 6 1
3
3 1 6 4 1 4
1
1
Algorithm: Initial label = infinity for all nodes, 0 for source; nodes unmarked Pick umarked node with smallest label [here: source] Update its children and mark it; mark/unmark shortest path links [Here: three nodes are updated (3, 6, 1), their links are marked.]
5
8
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
IP : OSPF
3 3 0 6 1 1 4 1 3 1 6
(3/8)
3 1 4 0 3 6 1 1
3 1 5 4 1 2 1 4 8
Algorithm: Initial label = infinity for all nodes, 0 for source; source marked Pick umarked node with smallest label [node with label 1] Update its children and mark it; mark/unmark shortest path links [nodes with label 5: umark link from source, mark link 1-5]
5
8
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
IP : OSPF
3 3 0 6 1 1 4 1 2 3 1 5
(4/8)
3 1 4 8 0 3 6 1 1
3 1 5 4 1 2 1 4 8 10
Algorithm: Initial label = infinity for all nodes, 0 for source; source marked Pick umarked node with smallest label [node with label 2] Update its children and mark mark/unmark it; shortest path links [label 10, mark links 2-10]
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
IP : OSPF
3 3 0 6 1 1 4 1 2 3 1 5
(5/8)
3 1 4 8 10 0 3 6 1 1
3 1
6 1 4 4 8 2 10
4 1
Algorithm: Initial label = infinity for all nodes, 0 for source; source marked Pick umarked node with smallest label [node with label 3] Update its children and mark it; mark/unmark shortest path links [update node from label 5 to 4, umark old link, mark new one]
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
IP : OSPF
3 3 0 6 1 1 4 1 2 3 1 4 6
(6/8)
3
1 4 8 10
3 1
6 1 4 4 8 2 8
3 0 6 1 1
4 1
Algorithm: Initial label = infinity for all nodes, 0 for source; source marked Pick umarked node with smallest label [node with label 4] Update its children and mark it; mark/unmark shortest path links [update label from 10 to 8, unmark/mark link]
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
IP : OSPF
3 3 0 6 1 1 4 1 2 3 1 4 6
(7/8)
3
1 4 8 8
3 1
6 1 4 4 8 2 7
3 0 6 1 1
4 1
Algorithm: Initial label = infinity for all nodes, 0 for source; source marked Pick umarked node with smallest label Update its children and mark it; mark/unmark shortest path links
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
IP : OSPF
3 3 0 6 1 1 4 1 2 3 1 4 6
(8/8)
3
3 1
6 1 4 4 8 2 7
1 4 8 7
0
3 6 1 1
4 1
Algorithm: Initial label = infinity for all nodes, 0 for source; source marked Pick umarked node with smallest label Update its children and mark it; mark/unmark shortest path links
(typo - the link from 2 to 7 should not be marked)
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
Border Gateway Protocol (BGP): Each router has to know about all other routers in the Internet? No - use "hierarchical routing" uci.edu calren2.net cenic.net S R R G R R R G R R G nox.org mit.edu D R G R G R R
R ucaid.edu
- Use OSPF within each AS, e.g. to get from S to UCIs gateway to calren2.net. - Use BGP to determine the sequence of autonomous systems from S to D.
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
BGP: Part (A): Exchange preferred routes with adjacent autonomous systems. Send message [route,delay] for each destination AS to each adjacent AS.
UCI
University of California, Irvine
EECS 161 - Introduction to Computer Networking
Scott Jordan
Internet / Routing
Part (B):
(Tanenbaum, fig. 5-55) Example: F to D - eliminate routes that include you (I,E) - eliminate routes that violate policies - among remaining, choose shortest path Policies may include economic arrangements between autonomous systems.